No one is saying it was. We know the graveyard scenes, just like the rest of the modern New York scenes were in 2012. We're just stating that Amy and Rory themselves were from after 2020.

And there's nothing in the ep. to suggest that.

Logically speaking, they kind of have to be. There's the aforementioned fact that in 2020 they go to Wales to visit wave at their younger selves as they embark on their adventure with the Silurians as seen in The Hungry Earth.

But even if we ignore that with some sort of hand wave, the Doctor dropped them off and then hid, they were gangers or Tessalectas or whatever, we still have Dinosaurs on a Spaceship. Rory tells his father he's 31, and sine Brian doesn't say anything like "you're only 23" we can assume that Rory really was born 31 years prior to that episode. Since we know from The Eleventh Hour and The Beast Below that Amy was 21 as of 2010 making her born in 1989 and we know from Let's Kill Hitler that Rory is the same age as her, that means Dinosaurs on a Spaceship hs to be set in 2020. Add onto that that The Power of Three spans a full year and Amy and Rory as seen in The Angels Take Manhattan can be from no earlier than 2021. It's all canon.

There's nbo such thing as canon on Doctor Who and in The Hungry Earth it was changed to only Amy standing on the hill since Rory was erased from time. And there's still no frame for the graveyard scenes. Of course since Chibnall wrote the Silurian two parter it's best to ignore it.

__________________
The greatest science fiction series of all time is
Doctor Who! And I'll take you all on, one-by-one
or all in a bunch to back it up!"
--- Harlan Ellison, from his introduction
to the PINNACLE series of Doctor Who books